Emissions of toxins into the atmosphere have become environmental issues of increasing concern because of the dangers posed to human health. For instance, coal-fired power plants and medical waste incineration are major sources of human activity related mercury emissions. Mercury emitted to the atmosphere can travel thousands of miles before being deposited to the earth. Studies also show that mercury from the atmosphere can also be deposited in areas near the emission source.
It is estimated that there are 48 tons of mercury emitted from coal-fired power plants in the United States annually. One DOE-Energy Information Administration annual energy outlook projected that coal consumption for electricity generation will increase from 976 million tons in 2002 to 1,477 million tons in 2025 as the utilization of coal-fired generation capacity increases. However, mercury emission control regulations have not been rigorously enforced for coal-fired power plants. A major reason is a lack of effective control technologies available at a reasonable cost, especially for elemental mercury control.
One technology that has been used for controlling elemental mercury, as well as for oxidized mercury, is activated carbon injection (ACI). The ACI process includes injecting activated carbon powder into the flue gas stream and using a fabric fiber or electrostatic precipitator to collect the activated carbon powder that has adsorbed mercury. Generally, ACI technologies require a high carbon to mercury ratio to achieve the desired mercury removal level (>90%), which results in a high cost for sorbent material. The high carbon to mercury ratio suggests that ACI does not utilize the mercury sorption capacity of carbon powder efficiently. Additionally, if only one particle collection system is used, the commercial value of fly ash is sacrificed due to its mixing with contaminated activated carbon powder. A system with two separate powder collectors and injecting activated carbon sorbent between the first collector for fly ash and the second collector, or a baghouse, for activated carbon powder, may be used. A baghouse with high collection efficiency may be installed in the power plant facilities. However, these measures are costly and may be impractical, especially for small power plants.
Since water-soluble (oxidized) mercury is the main mercury species in bituminous coal flue gas with high concentrations of SO2 and HCl, bituminous coal-fired plants may be able to remove 90% mercury using a wet scrubber combined with NOx and/or SO2 control technologies. Mercury emission control can also be achieved as a co-benefit of particulate emission control. Chelating agents may be added to a wet scrubber to sequestrate the mercury from emitting again. A chelating agent adds to the cost due to the problems of corrosion of the metal scrubber equipment and treatment of chelating solution. Elemental mercury is the dominant mercury species in the flue gas of sub-bituminous coal or lignite coal, and a wet scrubber is not effective for removal of elemental mercury unless additional chemicals are added to the system. It is undesirable, however, to add additional potentially environmentally hazardous material into the flue gas system.
Certain industrial gases, such as syngas and combustion flue gas, may contain toxic elements such as cadmium, chromium, lead, barium, beryllium, nickel, cobalt, vanadium, zinc, copper, manganese, antimony, silver, thallium, arsenic or selenium, in addition to mercury. Like mercury, these toxic elements may exist in elemental form or in a chemical compound comprising the element. It is highly desired that the presence of one or more of these toxic elements be substantially reduced before a syngas is supplied for industrial and/or residential use or before a gas is emitted to the atmosphere.
There is a genuine need of a sorbent material capable of removing mercury and/or other toxic elements from fluids such as flue gas and syngas, with a higher capacity than activated carbon powder alone. It is also desired that such sorbent material be produced at a reasonable cost and conveniently used.